A MIMO (Multiple-Input Multiple-Output) system has the highest potential for realizing high-speed transmission in 3G and 4G cellular systems, fixed wireless access system and wireless local network (Non-Patent Document 1). MIMO systems are broadly divided into two types: a spatial diversity system and a multiple access systems, and each has different characteristics and features.
In a spatial diversity system, the same data is transmitted through different antennas, and the statistics of the space fading characteristics of the antennas are independent, so that such a transmission scheme effectively suppresses space selective fading. STTD (Space-Time block coding Transmit Diversity) is one such system. STTD has a special orthogonal design and superior performance and is easy to be realized, so that STTD attracts widespread attention in the academic and industrial worlds. At present, STTD is the most studied and used space-time coding and modulation technique. This STTD technique was formally incorporated into WCDMA Proposal 3G TS 25.221 (Non-Patent Document 6) soon after the technique was first proposed. The employed coding scheme is a typical 2×2 space-time block code.
A multiple access system transmits serial-to-parallel (S/P) converted bit streams through different antennas. The multiple access system identifies different data transmitted at the same time utilizing the independency of different antenna radio channels. As a result, the system capacity can be increased. One example of a multiple access system includes a V-BLAST system.
The V-BLAST system has high capacity and has spatial diversity capability. Further, in order to identify the different data transmitted at the same time, the V-BLAST system requires the number of antennas on the receiving side equal to or larger than the number of transmitting antennas. However, as for a cellular communication system, demands for a size and portability of mobile telephones limit the number of antennas.
In order to solve this problem, Nortel proposes MPD (Multi-path Diversity) for 3GPP (Non-Patent Document 2, Non-Patent Document 3, Non-Patent Document 4 and Non-Patent Document 5). MPD has a throughput rate similar to V-BLAST, but does not require the number of receiving antennas equal to or larger than the number of transmitting antennas. In an MPD transmission system, as shown in FIG. 1, first, S/P converting section 81 performs S/P conversion on a code stream from a data source and obtains two code streams of first code stream s1 and second code stream s2.
Next, above first code stream s1 and second code stream s2 are branched, and first normalizing section 82 performs normalization processing on one of the branches. This normalization processing refers to, for example, processing such as multiplication of the codes of the first code stream and second code stream by constant 1/√2 so that the power of the transmitted code becomes 1. The normalized code streams are then subjected to spreading processing by first spreading section 83. For example, a specific spread code is multiplied by the code to be spread, and a first spread sequence and a second spread sequence of the first path are outputted.
Code streams of another branch are inputted to STTD coding section 84 and subjected to the processing expressed by following equation 1.
                    [        1        ]                                                                      (                                                                      s                  1                                                                                                      s                  2                                                              )                ⇒                  (                                                                      -                                      s                    2                    *                                                                                                                        s                  1                  *                                                              )                                    (                  Equation          ⁢                                          ⁢          1                )            
Code streams −s2* and s1* subjected to STTD coding by STTD coding section 84 are outputted to second normalizing section 85. Second normalizing section 85 performs normalization processing such as, for example, multiplication of above code streams −s2* and s1* by constant 1/√2.
The normalized code streams are then subjected to spreading processing by second spreading section 86. This spreading processing is processing of multiplying a specific spread code by the code streams to be spread. As a result of this processing, a first spread sequence and a second spread sequence of the second path are outputted.
Then, the first spread sequence and second spread sequence of the second path are delayed by one chip by delaying section 87. Subsequently, the delayed first spread sequence and second spread sequence of the second path are added to the spread first spread sequence and second spread sequence of the first path, respectively, by combining section 88. The sequences after the addition are transmitted through two antennas.
FIG. 2 is a block diagram showing a configuration of an MPD reception system. A signal received through the receiving antenna is inputted to despreading section 91 and subjected to despreading processing. That is, the inner product of the received signal is taken using the corresponding spreading code vector and the chip vector of a received code.    Non-Patent Document 1: T. S. Rappaport, A. Annamalai, R. M. Buehrer, and W. H. Tranter, “Wireless communications: past events and a future perspective,” IEEE Commun. Mag., vol. 40, no. 5, Part: Anniversary, pp. 148-161, May 2002.    Non-Patent Document 2: Nortel, “Multi-paths diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, R1-030565.    Non-Patent Document 3: Nortel, “Multi-paths diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, NY, R1-030760.    Non-Patent Document 4: Nortel, “Further results on Multi-Paths Diversity for MIMO (MPD)”, 3GPP TSG RAN WG1, NY, R1-031102.    Non-Patent Document 5: Nortel, “Rate Control for MPD”, 3GPP TSG RAN WG1, NY, R1-031316.
Non-Patent Document 6: 3G TS 25.221 V3.2.0 (2000-03) [online], available from http://www.3gpp.org.